Based on the derived cargo distribution, the recorded departure trim and freeboards, and the re-verified lightship weight, review of the barge's stability shows characteristics slightly inferior to those derived from the data available before the occurrence. The initial transverse stability remains positive, with the maximum righting lever attained at 8.1 heel, and deck edge immersion at midships occurring at 6.5 heel. The barge's intact stability was such that in static calm water conditions and with a secure cargo, a constant transverse heeling moment of some 8,000 ft-tons (2477 metre-tonnes) would be required to overcome its righting ability. However, in the dynamic conditions resulting from a combination of the barge's sea motions and the initial cargo shifting, the magnitude of a suddenly-applied heeling moment necessary to cause capsizing would be significantly lower. The barge's actual righting ability on departure was markedly lower than the minimum criteria specified. However, it should be noted that the SEASPAN240 was not required to comply with CCG STANDARD: STAB 8 (Interim Standard for the Intact Stability for Unmanned Barges). However, review of the barge's stability characteristics shows that if the barge had been loaded to the assigned (effective) freeboard of 6'2, with 10,000 long tons (10,160 tonnes) of deck cargo trimmed and distributed in a manner similar to that reported, the SEASPAN240 would have met the CCG minimum criteria, providing a much stronger resistance to capsizing than prevailed at the time of the occurrence. The maximum righting lever and total righting energy of the barge would have been respectively 2.7 and 5 times greater than when loaded. Capsizing usually occurs when a vessel loses transverse stability, and can be due to an individual cause or a combination of contributory causes. In the case of an intact and initially stable vessel, capsizing is often initiated by the transverse movement of a weight already onboard. The magnitude of the weight required depends on the athwartship distance through which it is moved, and also on the righting ability of the vessel at the time. Consequently, an initially small moment can induce a vessel with low transverse stability to heel, so that the sloping sides of a bulk deck cargo are tilted beyond their established angle of repose. The subsequent shift of cargo and resultant increase in heeling moment can cause the heeling to continue and accelerate, until transverse stability is suddenly overcome. Those attending the loading operation, or who were onboard the tug during the voyage, acknowledged, and further post-occurrence inspections and enquiries confirmed, that: the barge was virtually upright and trimmed slightly by the stern on departure; all bilges were virtually dry prior to and on completion of cargo loading, with no significant free-surface effect to adversely affect the barge's transverse stability; no underwater hull damage was incurred during the voyage, nor any asymmetrical flooding which could have initiated heeling; there were no reported actions due to high wind, sea or towrope to impose any significant or sudden heeling forces on the barge; there was only intermittent rainfall during the loading operation and throughout the voyage, and, consequently, little likelihood of any significant retention of rainwater within the cargo box area; and no shipped seas were seen to be retained on the barge, and no change of trim noticed prior to the sudden capsizing. Any adverse effects on the barge's stability from the above possibly contributing factors may be discounted as insignificant. It is deduced that the capsize was caused by a transverse shift of deck cargo. It is highly likely that the actual configuration of the limestone inside the cargo box was less uniform than the ideal reported. Some asymmetrical settling of the sloping faces of cargo, already at its natural angle of repose, would then be initiated by the barge's rolling motion. When the bow of the barge entered the stronger current off Admiralty Head, it sheared to starboard. The rolling motion was induced by the swell, in conjunction with the couple of the forces due to the current acting on the underwater hull and restraint of the towline acting on the upper part of the hull. Thus, a slight heel to port was initiated. Only a relatively small initial transverse shift of cargo would be needed to cause the deck edge to immerse and the barge to assume a small angle of heel. Subsequent rolling about this heeled angle would subject more of the sloping cargo surfaces to angles greater than their established angle of repose. Further transverse shifting would thus be caused. This sequence would continue at an accelerating rate until the barge's righting ability was overcome, resulting in the sudden capsizing reported by those on the tug.Analysis Based on the derived cargo distribution, the recorded departure trim and freeboards, and the re-verified lightship weight, review of the barge's stability shows characteristics slightly inferior to those derived from the data available before the occurrence. The initial transverse stability remains positive, with the maximum righting lever attained at 8.1 heel, and deck edge immersion at midships occurring at 6.5 heel. The barge's intact stability was such that in static calm water conditions and with a secure cargo, a constant transverse heeling moment of some 8,000 ft-tons (2477 metre-tonnes) would be required to overcome its righting ability. However, in the dynamic conditions resulting from a combination of the barge's sea motions and the initial cargo shifting, the magnitude of a suddenly-applied heeling moment necessary to cause capsizing would be significantly lower. The barge's actual righting ability on departure was markedly lower than the minimum criteria specified. However, it should be noted that the SEASPAN240 was not required to comply with CCG STANDARD: STAB 8 (Interim Standard for the Intact Stability for Unmanned Barges). However, review of the barge's stability characteristics shows that if the barge had been loaded to the assigned (effective) freeboard of 6'2, with 10,000 long tons (10,160 tonnes) of deck cargo trimmed and distributed in a manner similar to that reported, the SEASPAN240 would have met the CCG minimum criteria, providing a much stronger resistance to capsizing than prevailed at the time of the occurrence. The maximum righting lever and total righting energy of the barge would have been respectively 2.7 and 5 times greater than when loaded. Capsizing usually occurs when a vessel loses transverse stability, and can be due to an individual cause or a combination of contributory causes. In the case of an intact and initially stable vessel, capsizing is often initiated by the transverse movement of a weight already onboard. The magnitude of the weight required depends on the athwartship distance through which it is moved, and also on the righting ability of the vessel at the time. Consequently, an initially small moment can induce a vessel with low transverse stability to heel, so that the sloping sides of a bulk deck cargo are tilted beyond their established angle of repose. The subsequent shift of cargo and resultant increase in heeling moment can cause the heeling to continue and accelerate, until transverse stability is suddenly overcome. Those attending the loading operation, or who were onboard the tug during the voyage, acknowledged, and further post-occurrence inspections and enquiries confirmed, that: the barge was virtually upright and trimmed slightly by the stern on departure; all bilges were virtually dry prior to and on completion of cargo loading, with no significant free-surface effect to adversely affect the barge's transverse stability; no underwater hull damage was incurred during the voyage, nor any asymmetrical flooding which could have initiated heeling; there were no reported actions due to high wind, sea or towrope to impose any significant or sudden heeling forces on the barge; there was only intermittent rainfall during the loading operation and throughout the voyage, and, consequently, little likelihood of any significant retention of rainwater within the cargo box area; and no shipped seas were seen to be retained on the barge, and no change of trim noticed prior to the sudden capsizing. Any adverse effects on the barge's stability from the above possibly contributing factors may be discounted as insignificant. It is deduced that the capsize was caused by a transverse shift of deck cargo. It is highly likely that the actual configuration of the limestone inside the cargo box was less uniform than the ideal reported. Some asymmetrical settling of the sloping faces of cargo, already at its natural angle of repose, would then be initiated by the barge's rolling motion. When the bow of the barge entered the stronger current off Admiralty Head, it sheared to starboard. The rolling motion was induced by the swell, in conjunction with the couple of the forces due to the current acting on the underwater hull and restraint of the towline acting on the upper part of the hull. Thus, a slight heel to port was initiated. Only a relatively small initial transverse shift of cargo would be needed to cause the deck edge to immerse and the barge to assume a small angle of heel. Subsequent rolling about this heeled angle would subject more of the sloping cargo surfaces to angles greater than their established angle of repose. Further transverse shifting would thus be caused. This sequence would continue at an accelerating rate until the barge's righting ability was overcome, resulting in the sudden capsizing reported by those on the tug. The cargo deadweight and its configuration on departure was such that the barge did not retain sufficient intact transverse stability to withstand what was initially a relatively small shift of deck cargo. The trimmed and sloping surfaces of the cargo were such as to allow a transverse shift to occur at only a slight angle of heel. On departure, the barge was upright and trimmed slightly by the stern. No underwater hull damage or asymmetrical flooding were incurred during the voyage. The barge capsized in good weather when the current slewed it at right angles to the direction of the tow. Exemptions granted from regulatory provisions by virtue of its age and nature of its voyages within the treaty zone allowed loading of the barge such that its intact stability was below the limits set by the Interim Standard for Intact Stability for unmanned barges.Findings The cargo deadweight and its configuration on departure was such that the barge did not retain sufficient intact transverse stability to withstand what was initially a relatively small shift of deck cargo. The trimmed and sloping surfaces of the cargo were such as to allow a transverse shift to occur at only a slight angle of heel. On departure, the barge was upright and trimmed slightly by the stern. No underwater hull damage or asymmetrical flooding were incurred during the voyage. The barge capsized in good weather when the current slewed it at right angles to the direction of the tow. Exemptions granted from regulatory provisions by virtue of its age and nature of its voyages within the treaty zone allowed loading of the barge such that its intact stability was below the limits set by the Interim Standard for Intact Stability for unmanned barges. The SEASPAN240 capsized because the cargo deadweight and it's configuration on departure was such that the barge did not retain sufficient transverse stability to withstand a relatively small shift of deck cargo The shift of cargo and subsequent heeling were initiated by a combination of sea actions, pulling and slewing forces acting on the barge when positioned at right-angles to the tow.Causes and Contributory Factors The SEASPAN240 capsized because the cargo deadweight and it's configuration on departure was such that the barge did not retain sufficient transverse stability to withstand a relatively small shift of deck cargo The shift of cargo and subsequent heeling were initiated by a combination of sea actions, pulling and slewing forces acting on the barge when positioned at right-angles to the tow.